This invention relates to an extension telephone system utilizing key telephone sets.
A variety of single central office line key telephone systems have been proposed. In any system, however, a cord for connecting a main equipment to each key telephone set requires more than 20 conductors. Such a conventional system requires conductors for each of various functions such as control office line talking, internal talking, lamp indication, private or exclusion talking control, individual paging etc. Accordingly, about 20 conductors are required to connect the main equipment to each key telephone set, offering disadvantages from the standpoint of a complicated engineering work, engineering work cost, system economy etc. This prevents a wide acceptance of a key telephone system for a single central office line which is directed to houses or dwellings, particularly apartment houses or shops. Even if a plan is made for wiring a house for a key telephone system installation during construction the house, difficulty is experienced due to many conductors involved. In order to decrease the number of conductors required, introduction of a time division system etc. may be considered as a countermeasure. In this case, however, such a system requires a complicated circuit and difficulty is also encountered in reducing such a system to practice from the standpoint of system economy.
It may be said that an extension key telephone system for houses or shops is not necessarily required to perform the same functions as those of a conventional key telephone system. Where about three key telephone sets are connected to the main equipment, if an individual paging or calling is possible, a general voice paging or calling function is not necessary. Even where central office talking and internal talking lines are shared in a telephone system, if a preference is given to a talking over a central office line, the telephone system suffers no great inconvenience.